Advanced technology fibers, such as aramid and graphite or carbon, have substantial tensile strength for their weight and size which makes such fibers desirable for use in many engineering applications. Many applications of such fibers are well known, such as in the aircraft industry. Such fibers are often used in conjunction with a matrix binder to fix the fibers in particular orientations and to give form and structure to the intended device. One such use for such advanced fiber composites is for cables, tethers or other tension carrying devices. However, the advanced fiber composite cable is not easily secured at its ends. The problem is recognized in the art, but fully effective solutions have until now, been illusive.
There are a number of reasons for the difficulty in providing a reliable and satisfactory connection or termination for advanced fiber composite cables. For example, the fibers tend to be rather brittle in their transverse orientation in spite of their great longitudinal strength. Thus, clamping type devices which impose high compressive forces on the cable are not very suitable as they are likely to damage the fibers.
Another aspect of the difficulty for providing a connection termination for fiber composite cable is explained in U.S. Pat. No. 3,660,887 to Davis. In particular, the longitudinal load is not necessarily uniformly distributed across the cross section of the cable because of the weak bridging strength of matrix binders. As such, stress forces are concentrated on particular fibers, usually along the periphery thereof, which may exceed their strength limitations causing such over stressed fibers to break, and may eventually lead to catastrophic failure of the cable.
One well known technique for forming terminations for cables, in general, is called potting. Pottings are enlarged ends at the termination of a cable which enable a structural element to obtain a secure mechanical grasp to the cable. However, the bond formed between the potting material and the fiber composite cable is typically weaker than the cable. Thus, a fiber composite cable provided with a conventional potted termination will fail by the cable simply pulling out of the termination. As such, the design limitation of such cables is the termination and not the cable per se, which is converse to normal engineering practice. Several design arrangements for potting fiber composite cables have been disclosed such as in U.S. Pat. Nos. 3,283,380, 3,660,887 and 3,672,712. Such arrangements have not provided satisfactory solutions for composite cables and tethers and other tension carrying devices.
In addition to the foregoing problems for providing connections for advanced fiber composite cables, the cables are particularly prone to failure at the termination in arrangements where the cable is subjected to cyclic loading or cyclically applied forces.
Clearly, there is a need for an improved, more effective termination design for advanced fiber composite cables.
Accordingly, it is an object of the present invention to provide a connection termination for composite rods and cables that overcomes the drawbacks of the prior art as discussed above.
It is a more particular object of the invention to provide a connection termination for composite rods and cables which securely holds the rods in the connection termination and does not cause the rods to fail in the connection termination.